Convert Lambda into Java Function

Question

I have 2 questions.

1.

I want to convert Lambda into a Java function, but do not know how to do it.

//GET Method For REST Application
public Topic getTopics(String id){
    return topics.stream().filter(t -> t.getId().equals(id)).findFirst().get();
}

The topics is a list defined as:

private List<Topic> topics = new ArrayList<Topic>(
    Arrays.asList(
        new Topic("1","Spring Framework","Spring")
    )
);

Class Topic is defined as:

public class Topic {

    public String id;
    public String name;
    public String description;

    public Topic() {}

    public Topic(String id, String name, String description) {
        super();
        this.id = id;
        this.name = name;
        this.description = description;
    }

    public void setId(String id) {
        this.id = id;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getDescription() {
        return description;
    }

    public void setDescription(String description) {
        this.description = description;
    }

    public String getId(){
        return this.id;
    }
}

2.

I have 1 more question regarding the code. It is about filter function. Why is it that only t can be used as predicate? Hovering over t gives me this definition

Topic t - project_path.getTopics(...).() -> {...} Predicate.test(Topic)

t is simply a variable you give for each instance in the topic stream. You can change t -> t.getId().equals(id) into foo -> foo.getId().equals(id) if you want to. — Jai
– Jai, Commented Jun 19, 2018 at 5:04

Rea Sand · Accepted Answer · 2018-06-19 06:30:04Z

3

That lambda in your filter combinator can be extracted into a variable (predicate):

Predicate<Topic> hasSameId = topic -> id.equals(topic.getId());

or into a method:

Boolean hasSameId(Topic topic) {
    return id.equals(topic.getId());
}

In either case it would be good practice though to pass id along by currying either version:

Function<String, Predicate<Topic>> hasSameId = id -> topic -> id.equals(topic.getId());

It can then be used like this:

topics.stream().filter(hasSameId.apply(id)).findFirst();

edited Jun 19, 2018 at 6:30

answered Jun 19, 2018 at 5:49

Rea Sand

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Comments

isnot2bad · Accepted Answer · 2018-06-19 06:18:28Z

1

You definitely need to learn more about JAVA 8 streams. Anyhow here is answer to your questions:

public Topic getTopics(String id) {
    return topics.stream().filter(t -> t.getId().equals(id)).findFirst().get();
}

is same as

public Topic getTopics(String id) {
     for (Topic curTopic:topics) {
          if (curTopic.getId().equals(id)) return curTopic;
     }
     return null;
}

Secondly, for your second question, you can use anything as formal parameter, so below will also work fine (see also Syntax of Lambda Expression):

public Topic getTopics(String id) {
    return topics.stream().filter(p -> p.getId().equals(id)).findFirst().get();
}

Hope that helps you :)

edited Jun 19, 2018 at 6:18

isnot2bad

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answered Jun 19, 2018 at 5:05

Aman Chhabra

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2 Comments

Andreas Over a year ago

Not exactly same as, given that get() throws NoSuchElementException if nothing is found.

Aman Chhabra Over a year ago

@Andreas The motive here is to give basic idea. All boundary scenarios can be amended as and when required.

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